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AGAPITO
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Abstracts |
Numerical Modeling Abstracts (click to view abstracts)
Analysis of Fracture
in Rock and Rock Masses A rock fracture model is outlined which incorporates the specific work of fracture and the effective initial crack length as properties that can be used to predict the peak load in a rock structure. Experimental peak load results for red and grey granite for respectively five and seven different shape specimens are compared to the predictions of the rock fracture model; the maximum difference between predicted and observed results is less than 30% for the grey granite and about 30% for the red granite. The consequences of using the rock fracture model to analyze hydraulic fracturing of rock masses is discussed. The pressure versus crack length response for several situations with unequal far field stresses are presented for the case where the fluid pressure enters the fracture and, for the case when the fluid is restricted from entering the fracture, the pressure versus fracture length is discussed. It is concluded that in hydraulic fracturing a size effect on the breakdown pressure is dependent on the ratio of the principal stresses which act at infinity; this explains the apparent contradiction in some size effect studies in hydraulic fracturing. When the fluid pressure is restricted from entering the fracture, firstly the fluid pressure must eventually increase with fracture length, whereas when the fluid enters the fracture the pressure to cause fracture extension decreases with fracture length. And secondly, when a far-field compressive stress exists the fluid pressure must increase significantly with fracture length.
Analytical Investigation of Shaft Damages at West Elk Mine
(click to view entire
paper in PDF format) Several shear failures were observed in Shaft #1 at the Mountain Coal Company, LLC, West Elk Mine, after mining longwall Panel 23, 1,100 ft to the east of Shaft #1. It was speculated that this shear damage could be related to differential ground movement caused by in situ stress relief from the “stress shadow” of the caved zone above longwall Panel 23. A numerical study was conducted to assess the possibility of the shaft shear damage being caused by in situ stress relief and the potential for additional damage to Shaft #1 and two other nearby shafts, due to mining nearby longwall Panel 24. Three-dimensional (3D) models were built in FLAC3D to simulate past and future mining near shafts, the estimated local anisotropic and directional horizontal stresses, and the overlying variable surface topography. The numerical analyses indicated that stress relief due to mining Panel 23 caused the shear damage to Shaft #1and that additional damage to Shaft #1 and the other two shafts, would likely result from mining longwall Panel 24. Additional shear damage was documented in Shaft #1 when longwall Panel 24 was mined, confirming the results of the numerical analyses. |
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Project Summaries - Mineral |
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Project Summaries - Capability |
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Evaluation of Ground Control Requirements for D Orebody Load-Haul-Dump Block
Molycorp (click to view entire paper in PDF
format) Molycorp, Inc.’s Questa Mine plans to switch from gravity draw in Block 1 to a load-haul-dump (LHD) draw system in the East and West blocks of the D Orebody. Changes in mining method require modifying the support design. This paper summarizes the numerical modeling effort and findings in evaluating abutment stress conditions and ground support requirements for LHD mining. A three-dimensional, elastic-plastic, FLAC3D model was constructed comprising a detailed mesh representing the excavation geometry of the LHD drawlines encapsulated within a coarse, global mesh representing the entire D Orebody. The mining sequence was simulated to output stress changes at the LHD Level during advance of the caving front. The drifts at the LHD Level were mined before or after several key caving steps to represent pre-undercut and post-undercut conditions. A fictitious support pressure was then applied to the walls of the drifts and incrementally relaxed to measure convergence as a function of support pressure. LHD Level support requirements were determined from these ground-support interaction analyses. Conclusions were that a thick shotcrete liner would be required for the drifts in weak andesite, while light shotcrete and rockbolts would be sufficient in areas of strong aplite (granite porphyry).
Fracture Reopening During Hydraulic Fracturing Stress Determinations
This paper addresses the problem of the interaction of borehole and fracture fluid pressures on fracture reopening during pressurization cycles used to determine the maximum principal stress perpendicular to a borehole in impermeable rock. Our interest arose because of discussions on the interpretation of "hydraulic fracture" test results conducted at the Hanford Site, Richland, Washington. For these tests, the maximum principal stress was determined from the secondary breakdown pressure. Recognizing the pressure in the fracture is coupled to the borehole pressure and depends on the fracture aperture, the rate of pressurization, fluid properties, and the deformability of the fracture, a simple equation, such as this equation, is a crude approximation for a complex process.
Recently, Asgian (1988) completed a study of fluid flow in deformable,
naturally fractured reservoirs and during that study developed a computer
code, FFFLOW, to analyze coupled fluid flow/joint deformation problems. We
have applied that code to the fluid flow-fracture reopening problem
associated with interpretation of hydraulic fracturing stress
determinations. After defining the problem and presenting the results of a
numerical study, the implications for interpretation of hydraulic fracturing
tests for stress determination are discussed. Changes in operating
procedures are suggested to improve the interpretation of the fracture
reopening tests.
Horizontal
Stresses as Indicators of Roof Stability
(click to view complete paper in PDF
format) High horizontal stresses were recognized to impact roof stability more than 60 years ago. Since then, numerous measurements associated high horizontal stresses with difficult ground conditions. This paper presents case histories illustrating the practical usage of roof stress determinations for helping assess stability, not only in the case of high horizontal stresses but also of low stresses. Examples are given of high stresses associated with faults, mine design changes, quantification of stress shadow effect, and anistrophy. The paper concludes with a comparative evaluation on the effects of various stress fields on ground support requirements.
The Influence of Massive Sandstones in the Main Roof of Longwall Support
Loading (click to view complete paper in PDF
format) The significance of a massive sandstone unit in the immediate roof to longwall support weighting is well known. If the unit lies in the main roof above the zone of caving, its influence becomes more difficult to calculate using traditional methods. Much of the reserves at Cyprus Plateau's new, multi-seam Willow Creek Mine are located in the A Seam. The massive Kenilworth Sandstone is typically situated 21.3 m (70 ft) above the A Seam in the mine roof and is on average 14.6-m (48-ft) thick, ranging to 29.0-m (95-ft) thick. This paper describes the application of distinct-element modeling to determination of the appropriate yield capacity for supports at Willow Creek. The modeling was determined to be useful for understanding support loading mechanisms and as an aid to engineering judgment in selecting supports.
Modeling Block Cave Subsidence at the
Molycorp, Inc., Questa Mine Mine—A Case Study
(click to view complete paper in PDF
format) The evolution of surface subsidence is an important focus of study above Molycorp, Inc.’s newest block cave at the Questa molybdenum mine near Taos, New Mexico. The case study compares mature glory hole subsidence over the Goathill Orebody and subsidence emerging in its earliest stage over the new D Orebody block cave. Subsidence above the D Orebody was first detected in April 2003, 30 months after caving was initiated. Caving propagated to surface through 550 m of overburden at an average rate of 0.21 m per day. At the end of 2004, an average of 100 m of draw over a 1.4-hectare (ha) block produced a near-circular subsidence basin 5.8 m deep at its center and 90 m offset from the center of the block. Observations to date indicate a cave ratio of 10:1 and a gross cave bulking factor on the order of 10%. Historically, cave-angle projection models have been used to predict subsidence extents for reclamation planning. In light of evolving regulatory concerns, efforts are underway to develop a more accurate subsidence predictor using a three-dimensional (3D) numerical model. Particle Flow Code (PFC3D), a discontinuum “ball” code, was selected for modeling because of its ability to simulate stress fracturing of the rock mass and large-scale mass flow underground and at the surface, which are believed to be the dominant physical phenomena governing the formation of block cave subsidence. Advances simulating subsidence in the Goathill and D orebodies with PFC3D are discussed.
Ground Support Design Using Three-Dimensional Numerical Modeling at Molycorp,
Inc.'s, Block Caving Questa Mine
(click to view entire paper in PDF format) The Molycorp, Inc., Questa Mine, located in New Mexico, currently mines using a gravity-draw panel cave to extract molybdenum sulfide ore from the 600-m-deep D Orebody. Prior to initial development, geotechnical studies were undertaken to predict ground response for the design of entry support on the Grizzly and Haulage levels and in transfer raise connections. Heavy abutment pressures were anticipated ahead of the undercut, followed by significant stress relief as a consequence of a post-undercutting mining sequence. Detailed three-dimensional continuum modeling was conducted to predict changing stress states during the undercutting sequence and to evaluate the performance of various concrete and steel liner designs. Lithologic variation across the orebody was simulated and proved meaningful for identifying different stress transfer mechanisms and liner pressures in different types of squeezing ground. Recommendations for concrete liner thickness, concrete strength, reinforcement, and steel liner thickness were developed from modeling and, ultimately, were implemented during construction. Since the cave was initiated in October 2000, ground support has performed reliably, with only occasional compression cracking and minor tensile separation of the Grizzly Level liner in response to passing abutment loads. Observations to date corroborate model predictions and validate initial support design for the new deep orebody.
A Numerical Model of Fluid-Flow in Deformable, Naturally Fractured Rock
Masses A hybrid boundary element - finite-difference program, FFFLOW, has been developed for the modelling of fluid flow in deformable, jointed rock. The model has been used to simulate reservoirs with variable joint orientations, injection rates, joint dilation angles, and variable far-field stresses. The formulation and example results are presented.
Numerical Modeling of Rock Ridge Breakage in Rotary Cutting
(click to view entire paper in PDF format) In this paper, the finite element code, LS-DYNA 3D, was chosen as the simulation tool to study the rotary rock cutting process. By using an Automated Rotary Rock Cutting Simulator as the prototype, a numerical model of a continuous miner was developed. The rock grooves cut by the bits in the numerical model were used to investigate the rock ridge failure mechanism. The numerical study indicated that shear breakage acted as the dominant mechanism in ridge removal. Drum speed, depth of cut, and multiple bit interaction have minimal effects on the groove width. For a given bit pattern, Young’s modulus and Poisson’s ratio of the rock are two major parameters affecting the ridge removal.
A Numerical Study of Fluid Flow in a Deformable, Naturally Fractured
Reservoir: The Influence of Pumping Rate on Reservoir Response This paper describes a numerical study of fluid flow in a deformable, naturally fractured reservoir. The objective of the study is to determine how different pumping rates affect the response of the reservoir to fluid injections. The initial conditions in the reservoir are such that dilatant slip is triggered soon after the injections begin. The study shows that the peak response (peak fluid pressure, peak joint opening, peak joint aperture, and peak slippage) is less intense for lower pumping rates than for higher pumping rates when equal volumes of fluid have been injected at different rates.
Stability of Open Stopes Intercepted by Graphitic Shears at Ashanti
Goldfields Company Ltd., Obuasi Operations in Ghana
A geotechnical study was conducted to identify the cause(s) of stope instability at the Ashanti Mine (Obuasi, Ghana). As part of the investigation, laboratory testing of rock was performed and the rock mass was classified according to the Q-System (Barton et al., 1974). Numerical analyses were performed using the computer program FLAC (Itasca, 2001). The observed failure zone around the stopes was simulated using both Mohr-Coulomb and Hoek-Brown (Sheorey, 1977) models. Yielding along peripheral graphite bands and tensile failure were clearly identified to be the major contributors to stope instability. |
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